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What An Idiot is about when characters do things that are dumb, both in-universe and out-of-universe, when you think about their actions objectively; the character is usually acting out of character and denying all logic to do something that would move the plot along, even if they're usually smart enough to avoid things like this.

To answer your second question, Erin, did the character think not telling the person would be the better option? Or did they keep it secret for no good reason? If it's the latter, it's What An Idiot; if it's the former, it's not an example.

^ "No good reason" is such a blurry line that can easily be misused as to whether or not the character did cross it.

^ True; I think the line should be whether or not the character even has a justifiable reason at all or whether they're just defying logic and giving no reason for it. There are many reasons why a character wouldn't tell someone something (not all of them good, of course), but if the work does at least attempt to justify and rationalize the character's choice I don't think it would count unless it still manages to defy all logic. (such as X not telling Y something because they think Y is a traitor, even though Y has constantly proven themselves to be the most loyal team member)

Characters not telling each other important things to serve the plot is Poor Communication Kills. Where possible, the most specific trope for each event should be used.

Ah, true.

What An Idiot is when audiences see an intelligent decision they expect the characters to make, but they make a dumb one instead. Failed a spot check is not a decision so it's a different trope.

^ ...Y'know I think you just accidentally explained why this trope has so much misuse.

Characters doing dumb things is at the heart of most plots. It's not really noteworthy. What we do take note of is cases where a character (or sometimes an entire cast) becomes unusually stupid in order to create conflict.

• When a single character goes OOC to set the plot rolling, it's Idiot Ball.
• When a whole bunch of characters go OOC to set the plot rolling, it's Idiot Plot.
• When a particular character is always doing stupid things that cause problems for the others, they are The Millstone.
• When the other cast members could solve all their problems by casting out (or murdering, in extreme cases) The Millstone, you have an example of Just Eat Gilligan.

Note that all of these are standard tropes used by writers across the spectrum of media, because they are convenient. In comedies, they're also good for cheap laughs.

What An Idiot is an audience reaction, so it's not as easily quantifiable. What does need to be the case for examples to be valid is for the "smart" thing to be (a) within the capabilities of the characters, (b) something one would expect them to know or realize given their in-character knowledge, (c) consistent with their established characterization and role. In other words, the act (or lack of action, depending) must be out-of-character and obviously dumb.

Note that a lot of WAI examples are predicated on the characters needing to be Genre Savvy; for example, someone saying Let's Split Up, Gang! in a horror film is clearly asking for death, but the character themselves doesn't know they're in a horror film. This is invalid. Also, slasher flicks are generally dependent on everyone being dumb as horse droppings, so really shouldn't count.

When the Repair Shop gets cleaned out, we really need to send What An Idiot there to scrub the examples and make the actual meaning/usage of the trope more apparent.